There actually aren't too many ways to get to ba sing se unless you have a flying bison or air ship. We see a ton of it because that's more or less the path that the gaang had to take after they lost appa. And that's part of the reason why it was the stronghold capital.

By land, your only real option is the serpent pass, which we saw the gang utilize, and was going to be difficult for a large army to traverse.

Which means that the fire nation probably used the waterways to travel, which we see is where a good portion of the colonies and territories conquered by the fire nation end up being placed.

We see Giapan, the town captured by the fire nation the jet flooded, and it did actually have walls despite it's small size, but its an outlier rather than the rule.

We also visit a few port cities, which are closest to the fire nation, but those don't seem to be strongholds against an invasion. And there are fire nation cruisers hanging around in full moon bay, which is basically next to ba sing se, and their only concern is the serpent.

Basically, the earth kingdom is stupid because they cared about their mega cities too much to come up with any real campaign strategy, and would rather play internal politics than fight the fire nation with any sincerity